How do buffer solutions work?

Eg Acetic acid and sodium acetate:

  1. CH3COOH(aq) + H2O(aq) equilibrium_arrows H3O+(aq) + CH3COO-(aq)
  2. CH3COONa(aq) → CH3COO- (aq)
  1. Because acetic acid is weak it will not dissociate greatly so the equilibrium will be to the left leaving plenty of undisociated acetic acid.
  2. The sodium acetate will produce lots of extra ethanoate ions in solution. The extra ethanoate ions will have the added effect of pushing the equilibrium to the left, making an abundance of acetic acid.

These two factors are the basis of the carefully balanced equilibrium that allows buffer solutions to minimise pH changes when acid or alkali are added to them.

On addition of acid On addition of base
The extra H+ ions added to the solution will react with the ethanoate ions present to produce ethanoic acid. This is a reversible reaction but again because acetic acid is weak the H+ ions remain removed form solution stabalising the pH.
CH3COO- + H+ equilibrium_arrows CH3COOH
The acetic acid CH3COOH has provided the solution with plenty of H3O+ ions which will react with any hydroxide ions that are added to the solution. Therefore maintaining the pH.
H3O+ + OH- equilibrium_arrows 2H2O

Eg ammonia and ammonium chloride:

  1. NH3 + H2O equilibrium_arrows NH4+ + OH-
  2. NH4Cl equilibrium_arrows NH4+ + Cl-
  1. Because ammonia is a weak base it will stay mainly undissociated, leaving lots of ammnia molecules in solution
  2. Again the conjugate salt produces an excess of NH4+ ions. This again will push equilibrium (1) to the left giving an abundance of ammonia ions.
On addition of acid On addition of base
Extra hydrogen ions are removed by reacting with the excess ammonia ions:
NH3 + H+ equilibrium_arrows NH4+
They can also be removed by the hydroxide ions that are present from the reaction of NH3 with water.
OH- + H+ equilibrium_arrows H2O
Hdroxide ions added to the solution are removed simply by the ammonium ions.
NH4+ + OH- equilibrium arrowsNH3 + H2O

On the next page we will see how to calculate exactly what pH the solution will buufer at by knowing the pKa of the acid or alkali used and the concentrations of them.

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Author: Christopher Saywell (document modification date: 7th April 2004)